1. Technical Field
The present invention generally relates to data communication networks and, more particularly, to a vehicle multi-access communications network with enhanced transmitter interfacing for realizing improved current sourcing.
2. Discussion
Modern automotive vehicles are commonly equipped with multiple-access serial data communications networks to enable data transfers between various electronics within the vehicle. The Society of Automotive Engineers (SAE) has established a Standard J1850 Class B data communications network that has become widely accepted throughout the automotive industry. The standard J1850 is a set of technical requirements and parameters which specify the use of symbols for communicating serial data over a communications bus.
Input/output (I/O) bus transceivers are generally equipped with the vehicle electronics and interface with the communications bus to transmit and receive data. In automotive applications, variable pulse width modulation (VPWM) encoded signals are widely used. With VPWM encoded signals, a symbol includes a voltage logic level that extends for a period of time and then a voltage transition or edge. Generally speaking, the amount of time and the voltage level between trip points of the previous edge and the current edge defines the meaning of the symbol.
In communications networks where multiplexing schemes transfer pulse signals from one node (i.e., electronic module) to another, ground loops often become a source of noise. Noise results especially when large distances separate multiple ground points and during employment of low-level analog circuits. It becomes necessary to provide some form of discrimination or isolation against ground path noise and direct current offset voltages.
Grounding of nodes in multinode systems occur at different points resulting in potential differences between grounds; e.g., chassis ground and signal ground. Grounding components in vehicles necessitate this grounding approach. This usually causes unwanted noise voltages in the network. The magnitude of signal levels compared to noise voltages in the network provides a signal-to-noise ratio. If the signal-to-noise ratio affects network operations, then effort to improve the ratio must take place.
Past noise reduction remedies included two basic approaches: 1) avoid the ground loops by removing one of the type grounds and converting the system to a single-point ground network. (This provides an impractical remedy since DC currents cause voltage drops, and since additional circuit components are required at an added cost); 2) eliminate or minimize the effects of multiple grounds by isolating the two or more circuits. Isolation can be achieved by using: 1) transformers, 2) common-mode chokes, 3) optical couplers and 4) balanced circuitry.
Such noise reduction techniques yield favorable results, but these techniques require large numbers of isolating components. Such components are costly and may introduce other adverse effects which are entirely unanticipated. In an effort to minimize isolation problems in multinode networks, a search took place to find other means of isolating ground loops in multinode networks.
Additionally, each node or electronic device generally must have its own input/output bus transceiver to interface with the communications bus. Each input/output bus transceiver contains a transmitter circuit with a bus driver for transmitting data onto the vehicle communications bus. Conventional transmitter circuits with bus drivers usually operate to provide a preset voltage potential to the bus. With the multiple-access bus, the transmitting circuit drives the bus when the transmitter output voltage exceeds the voltage potential seen on the bus. As between multiple nodes, the node outputting the higher voltage potential generally drives the bus. When a transmitter circuit and associated bus driver applies a voltage, the bus driver also sources current from the bus driver onto the communications bus.
However, vehicular communications networks are often subjected to various noises sources which induce noise onto the data line, often through the grounding lines. On the one hand, ground noise can induce a change in the bus voltage. Also, on the other hand, changes in current on the bus can inject noise into surrounding electronics. The induced voltage is generally a function of the mutual inductance between two circuits and the rate of change of current. This principle is disclosed on page 38 of "Noise Reduction Techniques in Electronic Systems", 2nd edition, by Henry Ott. This induced voltage also causes changes in sourcing current on the bus. Conventionally, current changes can occur almost instantaneously and are often measured as current spikes. Such drastic current changes can cause an abrupt imbalance between the interfacing electronic devices and allow for relatively large amounts of noise to be induced onto sensitive electronics.
It is therefore one object of the present invention to provide for a vehicle communications network with multiple electronics interfacing and enhanced ground translation circuitry.
It is another object of the present invention to provide for a multi-access vehicle communications network which reduces induced noise effects, especially noise associated with nearby magnetic fields.
It is a further object of the present invention to provide for such a multi-access vehicle communications network which interfaces a plurality of transmitter circuits with bus drivers and reduces the rate of change of sourcing current on the communications bus.